Electrostatic discharge (“ESD”) protection structures are needed for integrated circuits. In ESD protection, the ESD circuit provides a path to bypass current from the terminal to a ground or from the terminal to a power supply rail, so that the current due to an ESD event bypasses the internal circuitry. Voltages far in excess of the normal operating voltages, in both positive and negative magnitudes, are observed during short duration electrostatic discharge events. The ESD protection structures prevent the corresponding ESD current from destroying sensitive components in an integrated circuit. Without ESD protection the devices coupled to the power terminal would fail.
ESD structures are often formed from so-called “parasitic” transistors that are formed when MOS or field oxide (FOX) devices are fabricated over a semiconductor substrate. For example, when a MOS device is formed having a gate terminal over a gate dielectric that overlies the substrate, source and drain diffusion regions are formed on opposite sides of the channel region beneath the gate terminal. Because the source and drain regions are of opposite conductivity type to the substrate, a lateral NPN or PNP bipolar transistor forms with its base in the substrate or well region. This device provides a current path that may be used as an ESD circuit. Silicon controlled rectifier (SCR) circuits of two bipolar parasitic transistors are also used as ESD structures.
Existing ESD protection circuits have relatively slow turn on speed and relatively high trigger voltages. As the semiconductor devices produced become increasingly smaller, the thin gate oxides formed become ever more susceptible to ESD. Improved ESD circuits are needed with low trigger voltages, without substantially increasing silicon area.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the illustrative example embodiments and are not necessarily drawn to scale.